The UMTS network architecture consists of three components, namely the Core Network, the UMTS Terrestrial Radio Access Network (UTRAN) and the User Equipment (UE). In UTRAN, three protocol layers are specified. These comprise: Layer 1, the physical layer which uses WCDMA on the radio link interface; Layer 2, the link layer containing Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Convergence Protocol (PDCP) sub-layers; and Layer 3, the Radio Resource Control (RRC) which exists in the control plane only. These layers are mirrored in the UE at the other end of the radio link with the UTRAN.
The physical layer transforms radio frames between the Node B and the UE. One radio frame, which may include several RLC Packet Data Units (PDUs), is sent during each Transmission Time Interval (TTI). Through an attribute of the transport format, the MAC layer decides by means of a Transport Format Combination (TFC) selection process which PDUs to send in each TTI. Three types of service are provided by the RLC protocol comprising Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM). The RRC is responsible for the transport format and also determines the transmission mode of the RLC.
The RLC performs segmentation/reassembly of RLC Service Data Units (SDUs), into/from smaller RLC PDUs. The three RLC operations needed to make flow control more efficient are a polling mechanism, a status transmission mechanism and a SDU discard mechanism.
The SDU discard function allows the sender (UTRAN or UE) to discard RLC SDUs when the transmission of the RLC SDUs does not succeed for a period of time or for a number of transmissions. The SDU discard function therefore helps avoid buffer overflow which might result in following RLC SDUs being discarded as a result of the buffer being full. There are several alternative operation modes of the RLC data discard function. Upper layers control which operation mode is used for each RLC entity. The data discard operation modes include, for example, “timer based discard with explicit signalling” applicable to AM transmission only and “timer based discard without explicit signalling” applicable to TM and UM transmission. SDU discard function is always configured for AM transmission, but may not always be configured for TM and UM transmission.
SDUs buffered in the RLC layer which are to be discarded should not, however, be discarded where the MAC layer has started its TFC selection. When TFC selection has started, the RLC layer delays discarding the data until the end of the next TTI. The problem encountered with this process is that the RLC layer is not made aware of when the MAC layer has started its TFC selection. The RLC layer will receive an indication from the MAC layer when a TFC selection has ended and the RLC layer is required to provide the MAC layer with data for the corresponding TTI. However, when no data from the RLC layer is to be transmitted in that corresponding TTI, the MAC layer does not send any indication of end of TFC selection to the RLC layer. This may occur for a number of consecutive TTIs and thus, since the RLC layer does not receive any indication from the MAC layer of end of TFC selection for the series of TTIs, the RLC layer cannot discard the data during that time. This impacts the Quality of Service (QoS) offered by the RLC layer as data will not be discarded as specified. Also, new data being received in the RLC layer from upper layers may be discarded as a result of RLC buffer overflow whilst the RLC layer awaits an indication from the MAC layer of end of TFC selection.